In general, calcium carbonates are mainly classified into “natural calcium carbonate” prepared from natural limestone, weathered shell or the like by mechanically grinding and classifying it and “synthetic calcium carbonate” (precipitated calcium carbonate) prepared from limestone by chemically reacting it. Known processes for synthesizing the synthetic calcium carbonate include the carbonation process, the lime-soda process, and the Solvay process, among which the lime-soda process and the Solvay process are sometimes employed for special applications while industrial synthesis of calcium carbonate typically involves the carbonation process.
Synthesis of calcium carbonate by the carbonation process involves reacting quick lime and carbonic acid gas, and typically comprises a slaving step in which water is added to quick lime CaO to give slaked lime Ca (OH)2, and a carbonation step in which, carbonic acid gas CO2 is injected into the slaked lime to give calcium carbonate CaCO3. At present, various techniques for controlling the particle shape or particle size or the like of the product calcium carbonate have been proposed by regulating reaction conditions in synthesis steps of calcium carbonate, particularly the carbonation step.
Various techniques for depositing calcium carbonate on fibers such as pulps have also been proposed. JPA 1994-158585 describes complexes comprising crystalline calcium carbonate mechanically bonded on fibers. On the other hand, U.S. Pat. No. 5,679,220 describes a technique for preparing a complex of a pulp and a calcium carbonate by precipitating the calcium carbonate in a suspension of the pulp by the carbonation process. U.S. Pat. No. 5,665,205 describes a technique for improving the brightness and purity of a waste paper fiber by adding a large amount of a filler for papers and paperboards to the fiber, which comprises sending a slurry of a waste paper pulp to a gas-liquid contactor where the pulp is broken by contact with a slurry of an alkali salt in a counter-flow direction in a contact/breaking zone and sending a suitable reactive gas and mixing it with the precipitating filler to deposit the filler on the surface of the fiber.
In addition, JPA 2013-521417 and US Patent Publication No. 2011/0000633 disclose techniques for preparing fiber webs in which calcium carbonate has been incorporated efficiently by depositing calcium carbonate in the step of forming the fiber webs (wet paper).